<p class="msr_btn04">
  <a href="#">button02</a>
</p>

.msr_btn04 a {
  background-color: #FFFFFF;
  border: 2px solid #1B73BA;
  border-radius: 3px;
  box-sizing: border-box;
  color: #1B73BA;
  display: block;
  font-size: 13px;
  padding: 13px 10px 13px 30px;
  position: relative;
  text-align: center;
  text-decoration: none;
  transition: 0.2s ease-in-out;
  -o-transition: 0.2s ease-in-out;
  -moz-transition: 0.2s ease-in-out;
  -webkit-transition: 0.2s ease-in-out;
  width: 230px;
}

/* 青の三角形 */
.msr_btn04 a:before {
  border: 6px solid transparent;
  border-left: 6px solid #1B73BA;
  box-sizing: border-box;
  content: "";
  display: inline-block;
  height: 12px;
  left: 15px;
  margin:-6px 0 0;
  position: absolute;
  top: 50%;
  transition: 0.2s ease-in-out;
  -o-transition: 0.2s ease-in-out;
  -moz-transition: 0.2s ease-in-out;
  -webkit-transition: 0.2s ease-in-out;
  vertical-align: middle;
  width: 6px;
}

/* マウスオーバー */
.msr_btn04 a:hover {
  color: #FFFFFF;
  background:#1B73BA;
}
.msr_btn04 a:hover:before {
  border-left: 6px solid #FFFFFF;
}